What limitation exists regarding subtype hierarchies?

A subtype entity can have only one parent entity because subtype hierarchies are designed to create a single inheritance model. In this model, each subtype inherits characteristics and behaviors from a single parent type, which helps maintain a clear and manageable structure for the data model. This prevents duplication of attributes and behaviors, ensuring that the subtype can effectively extend the functionality of its parent without ambiguity.

In this context, the limitation of having a single parent entity leads to a straightforward and easily understandable hierarchy, essential in many programming and data modeling situations. It allows the system to enforce rules consistently and ensures that the relationship between subtypes and their parents is well-defined.

The other concepts relating to multiple parent entities, referencing subtypes, and unique identifiers, do not apply here as they do not align with the principles of single inheritance in subtype hierarchies. The importance of having one parent ensures that the relationship remains clear and that the subtype appropriately represents a specific specialization of the parent entity.